Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. The appended claims are not intended to be limited, however, to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing or for sensing electrical signals produced by the heart or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled or wound conductor surrounded by an insulating tube or sheath couples the connector pin at the proximal end with the electrode at the distal end.
The purpose of the invention is to improve the reliability and manufacturability of a laser weld joint between a multi-wire winding and a connector. To achieve this, the winding is welded to a ring as a subassembly. The winding could be welded to the ring circumferentially or using a spot weld technique. The subassembly is subsequently welded to the connector during lead assembly.
The common weld joint comprises a winding screwed onto a cylindrical connector. The very last wind, or terminal end of the winding, are positioned against the shoulder of the connector. The shoulder and the terminal end of the winding are welded together (see FIG. 1).
It is challenging for the designer to create optimal welds when no transition exists between the wire ends and the connector. A number of problems are typically associated with connections between wound elements. For example:
1. The geometry of the "shoulder" design of the connector is such that it allows a weld joint to be formed from an unequal amount of material from each part joined. When unequal amounts of material are used to form a weld joint, the strength of the resultant joint is not repeatable. PA1 2. It is difficult to determine the position where the laser beam should be applied to provide the proper amount of energy to each part (i.e. the laser beam is positioned more on the connector than on the wire because the connector requires more energy than does the wire). PA1 3. Current configurations require that the conductor coil be expanded onto the connector ring which increases the outer diameter of the conductor coil and the size of the finished product. PA1 4. The strength of the weld is sensitive to the position of the wire ends with respect to the connector (gaps between components). PA1 5. The design is expected to be welded on the production line during lead assembly. It is difficult to use as a subassembly which would allow for an efficient manufacturing flow. PA1 6. Another problem associated with connections between wound elements and mating components in present day lead assemblies arises from the use of different alloys for the wound elements and mating components. Since dissimilar alloys have different melt temperatures and other thermal properties, such connections are difficult to weld. Moreover, as lead sizes decrease, problems of manufacturability arise. This is particularly true where crimping is employed to secure the wound component to a mating element.
During welding in a typical joining operation, a laser beam applies a specific amount of energy to both the wire and the connector. Because the wire has less mass than the connector, the wire accumulates heat very quickly and melts easily. The connector, which has more mass, "draws" the heat out of the weld area, and does not melt to the same extent as the wire. This makes it difficult to melt the required amount of metal to fuse components together. Because of a lack of melted metal from the connector, the wire "necks down" as it spreads over the connector. The "negative weld reinforcement" from a lack of melted material from the connector reduces the strength of weld joint.
Therefore, the connector requires more laser energy to melt than does the wire. To achieve a weld of optimal strength, the beam energy must be balanced between the connector and the wire. The proper beam targeting requires placement of the laser beam not equally on the joint such that more energy is on the shoulder side than on the wire. It is difficult for the line operator to target the laser beam on the joint properly and consistently.
When joining parts consisting of different materials, the difference in material thermal properties magnifies the energy balance problem. For example, platinum requires much more energy to melt than MP35N. To equally melt the components of a joint consisting of MP35N wire and a platinum connector, a larger portion of the energy provided will need to be transferred to the platinum part.
The connector design requires that the conductor coil is expanded onto the connector. It is placed onto the connector in order to hold the coil relative to a shoulder of the connector. This expansion increases the outer diameter of the conductor coil which ultimately increases the size of the finished product.
A weld joint is also very sensitive to the position of the wire ends with respect to the connector when the parts to be joined are of significantly different masses. At least two surfaces of the connectors, the underlying connector and the shoulder, are in contact with the wire. Because an unequal amount of energy needs to be applied to the parts, the relative position of the parts affects the location where the laser beam energy should be applied. If either or both of these surface conditions for a particular wire changes, such as a gap between the wire end and the shoulder, the position of the laser beam needs to compensate for the gap for an optimal weld to be produced. It is very difficult to quantify all the surface conditions that exist for each wire to be welded. With so many position variables, it is difficult to make weld joints with consistent performance.
In many cases, the connector is used as a transition from a coil to another coil. The design of connectors requires the preparation and assembly of one coil onto the connector followed by the preparation and assembly of the other coil to the same connector. This is a time-consuming process that does not lend itself to fabrication of subassemblies for use in an efficient manufacturing flow.
Typical of the prior art relating to joints between windings and connectors is U.S. Pat. No. 4,953,564 which discloses a cardiac pacing lead having an extendible fixation helix electrode that is mechanically and electrically connected to a rotatable conductor coil by squeezing the helix and coil together between a crimping sleeve and a crimping core. As the sizes of body implantable leads and their constituent parts become smaller, crimping becomes more difficult because the crimping tools cannot be made sufficiently small. Moreover, the same number of lead windings are not always subjected to the crimping action so that failure stress differs from lead to lead.
Some other selective examples of the patented prior art which can be mentioned briefly include U.S. Pat. No. 5,569,883 to Walter et al. which discloses laser welding a wire coil to an intermediate ring or the like. U.S. Pat. No. 5,571,146 to Jones et al. discloses laser welding dissimilar materials by means of an aperture within a lead. U.S. Pat. No. 5,385,578 to Bush et al. discloses laser welding a wire coil to a sleeve.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.